1. Field of the Invention
The present invention relates to a method of correcting color image data, and more particularly, it relates to a method of correcting color image data obtained by a color separation apparatus such as a process color scanner, thereby to obtain a proof image on a display medium such as a color CRT or a color film for previewing a printed image.
2. Description of the Prior Art
In multicolor printing, inks of the four colors yellow (Y), magenta (M) and cyan (C) (i.e., primary chromatic colors), and black (K) (i.e., an achromatic color for mainly compensating gradation of a dark part) are printed over each other to obtain various colors. Thus, the final color tone of a printed matter substantially depends on the ratios and influences the volumes of inked color components Y, M, C and K.
Accordingly, it is important to preadjust the ratios of the color components at desired values, and an apparatus for monitoring a reproduced image having the color components Y, M, C and K on a color CRT or a color film in advance to printing has been developed. In such an apparatus, the respective color components Y, M, C and K supplied from a color scanner are converted into color image data consisting of red (R), green (G) and blue (B), the latter colors comprising the three primary colors of the additive color mixture used in the color CRTs, or into another color image data consisting of Y', M' and C', which are three primary colors of the subtractive color mixture in color film, wherein the achromatic color component K is substantially added to respective primary chromatic color components Y', M' and C' for example.
In this case, a problem is caused due to what is referred to as the "additivity failure" characteristic. FIG. 1 shows a relationship between actual density of a multicolor print and the simple sum of density values of respective single-color prints. The "additivity failure" is such that the density of an actual print obtained by overprinting a plurality of color inks with each other is lower than the simple sum of density values of single-color prints. Such additivity failure characteristic is recognized in printed matter but is not present in a color CRT or a color film. Therefore, when the achromatic color component K is simply added to the respective chromatic color components Y, M and C to reproduce a proof image, the density of the proof image is different from that of an actual printed product.
In order to obtain a proof image which is faithful to the reproduced image through a compensation of the additivity failure, correction methods based on a correlation formula expressing the additivity failure have been developed. One of these is a technique using the well-known Neugebauer's equation.
However, since the additivity failure characteristic is influenced by scattering of light in printing paper or the like, the addivity failure characteristic expressed by the correlation formula does not always coincide with that in actual printing.
Further, since different correlation formulas should be prepared for obtaining proof images corresponding to different printing methods (e.g., gravure printing, offset printing, and those with different screen patterns), respectively, the process for compensating for the additivity failure is complex.